1. Field of the Invention
This invention relates to gravel packing of subsurface wells and has particular application to gravel packing in subsurface wells as used in the petroleum industry.
2. Description of the Prior Art
The production of fluid materials from an earth formation frequently carries with it portions of the formation causing potential problems with the producing equipment within the well bore and with the surface equipment associated with the well. Unconsolidated formations, particularly those containing loose sands and soft sandstone strata, present constant problems in well production due to migration of loose sands and degraded sandstone into the well bore as the formation deteriorates under the pressure of flow of fluids therethrough. This migration of particles may eventually clog the flow passages in the production system of the well and can seriously erode the equipment used in fluid production. The migration of particles is not necessarily dependent upon the viscosity or the flow rate of the produced fluids in that it is more likely to be dependent upon the degree of consolidation of the formation from which the fluids are produced and the procedures that have been used in preparing the formation for fluid production. In any event, the flow or migration of particles with the produced fluids may clog the production system and may lead to a complete cessation of flow or at least a reduction in production to a noneconomic rate.
It has long been known to control sand migration in a well bore by placing a pack of gravel on the exterior of a perforated or slotted liner or screen which is positioned within the well bore and through the unconsolidated interval of the formation being produced. A pack is placed in the annulus between the liner or screen and the open hole of the drilled formation. In prior art methods, the gravel is carried into the subsurface well in the form of a slurry, the carrier fluid being removed and returned to the surface. It has been known to place gravels of various size distributions to effectively halt sand migration through the gravel pack, the aperture in the liner or the screen being gauged so that the gravel material is packed within the well and will settle out in the annulus between the liner and the formation while the slurry carrying the fluid enters through the slots in the liner or screen and is returned to the surface.
In some cases, a gravel pack is placed between a liner or screen and an exterior casing which has been placed in the formation after the well has been drilled. The casing is perforated or slotted to permit production and the gravel material is placed in the annulus between the casing and the liner or screen without concern for damage to the formation.
Gravel packing is also used in uncased wells where the gravel is placed between a slotted liner or screen and the drilled formation. The packing of such wells can be complicated if the formation is susceptible to lost circulation or thief zones or if the formation may be damaged by conventional drilling or slurry fluids which may be used in the carrying of the gravel material to the subsurface location. In such wells, it is important to use a gravel carrier fluid which will not damage the formation and which will efficiently carry the gravel material from the earth's surface into the area to be packed.
It is also important in gravel packing a subsurface well that the gravel be placed in the annulus, whether between the liner and the formation or between the liner and the casing, at a density which provides an effective deterrent to the migration of formation particles while at the same time provides a desirable permeability for the flow of formation fluids into the well bore through gravel and into the screen or liner.
The prior art has recognized the need for tailoring both the gravel pack material and the carrier fluid to the particular characteristics of the formation being packed; however, so far as the present inventor is concerned, it has not been known to use a gas/liquid foam for the use of carrying gravel packing particulate material from the earth's surface into the subsurface formations for producing the desired well bore gravel pack. It is further believed that the methods and apparatus described in the present application have been heretofore unknown for placing of an effective gravel pack in a subsurface earth formation.